Today, Amazon.com announced Kindle Cloud Reader, its latest Kindle
reading application that leverages HTML5 and enables customers to
read Kindle books instantly using only their web browser - online
or offline - with no downloading or installation required. As with
all Kindle apps, Kindle Cloud Reader automatically synchronizes
your Kindle library, as well as your last page read, bookmarks,
notes, and highlights for all of your Kindle books, no matter how
you choose to read them. Kindle Cloud Reader with its integrated
touch optimized Kindle Store is available starting today for
Safari on iPad, Safari on desktop and Chrome at
www.amazon.com/cloudreader.

Conspicuously absent: Firefox and the iPhone. (What is Mozilla’s excuse for Firefox being so far behind Webkit in HTML5 offline web app support?)

Immediate consensus seems to be that this is Amazon’s response to Apple’s new rules, preventing them from linking to the Kindle store within the native iOS app.

I think Amazon had this in the works for a long time — a web-based Kindle reader has been around for a while, and it makes sense to improve it in these ways. But surely Apple’s new App Store rules for paid content have motivated Amazon to push harder in this direction.

The iPad version is especially good because the store is fully
optimized for the device. And you can easily switch back and forth
between the store and your own library. It feels like a native
app, but it’s not. You can even swipe back and forth to move
between pages (though it is a bit slow).

I wouldn’t go so far as to say it feels like a native app. Cloud Reader works great on the iPad by the standards of web apps, but the native Kindle iPad app is far more responsive, and has a far less cluttered interface simply because it isn’t surrounded by an extra layer of Safari UI chrome. The native app is more immersive. (You can eliminate the Safari UI chrome if you save Kindle Cloud Reader as a home screen web app, but even then it doesn’t feel as smooth as the native app.)